Dual phase steel comprising ferrite, a relatively soft phase and martensite/bainite, a relatively strong phase, are produced by annealing at temperatures between the A.sub.r3 and A.sub.r1 transformation points, followed by cooling to room temperature at rates ranging from air cooling to water quenching. The selected annealing temperature is dependent on the the steel chemistry and the desired volume relationship between the ferrite and martensite/bainite phases.
The development of low carbon and low alloy dual phase steels is well documented and has been the subject of extensive research in the metallurgical community; for example, conference proceedings on "Fundamentals of Dual Phase Steels" and "Formable HSLA and Dual Phase Steels", U.S. Pat. Nos. 4,067,756 and 5,061,325. However, the applications for dual phase steels have been largely focused on the automotive industry wherein the unique high work hardening characteristics of this steel are utilized for promoting formability of automotive sheet steels during pressing and stamping operations. Consequently, dual phase steels have been limited to thin sheets, typically in the range of 2-3 mm, and less than 10 mm, and exhibit yield and ultimate tensile strengths in the range of 50-60 ksi and 70-90 ksi, respectively. Also, the volume of the martensite/bainite phase generally represents about 10-40% of the microstructure, the remainder being the softer ferrite phase. Furthermore, the one factor that has limited their widespread application is their rather strong sensitivity to process conditions and variability, often requiring stringent and tight temperature, and other processing to maintain their desirable properties. Outside these rather tight processing windows, most of the steels of the state of the art suffer rather dramatic and precipitous drop offs in properties. Because of this sensitivity, these steels cannot be produced in a constant fashion in practice, thus, limiting their production to a handful of steel mills worldwide.
Consequently, an object of this invention is utilizing the high work hardening capability of dual phase steel not for improving formability, but for achieving rather high yield strengths, after the 1-3% deformation imparted to plate steel during the formation of linepipe to .gtoreq.100 ksi, preferably .gtoreq.120 ksi. Thus, dual phase steel plate having the characteristics to be described herein is a precursor for linepipe.
An object of this invention is to provide substantially uniform microstructure through the thickness of the plate for plate thickness of at least 10 mm. A further object is to provide for a fine scale distribution of constituent phases in the microstructure so as to expand the useful boundaries of volume percent bainite/martensite to about 75% and higher, thereby providing high strength, dual phase steel characterized by superior toughness. A still further object of this invention is to provide a high strength, dual phase steel having superior weldability and superior heat affected zone (HAZ) softening resistance.